1. Field of the Invention
The present invention relates to an apparatus for receiving burst data, and in particular relates to an apparatus for compensating for the degradation of a pulse width generated after being amplified to an original digital level.
2. Description of the Related Art
Lately, in order to improve the efficiency of data communications, an information exchange system for exchanging data in a data format called a xe2x80x9cpacketxe2x80x9d or xe2x80x9ccellxe2x80x9d has been extensively developed. In the packet or cell format, data are divided into certain lengths, and are transmitted in a burst, if necessary.
Since this burst data (data transmitted in a burst in the format of a packet or cell) of each packet or cell changes because of the transmission environment and conditions, the output which is amplified back to an original digital level in a relay or receiver unit (2R output: output obtained by performing the reshaping and re-timing of an original signal) of each packet differs in the degree pulse width degradation. Accordingly, it is necessary to compensate for the degree of degradation in order to exchange the burst data.
Conventionally, in the transmission of the burst data, attention was focused on the suppression of the degree of pulse width degradation caused during transmission, and the pulse width was regenerated using a flip-flop circuit, etc. when applying a clock signal (3R; performing the reshaping, re-timing and regeneration of an original signal) without compensating for the pulse width degradation generated after being amplified back to an original digital level (2R output). Accordingly, when the data pulse width is degraded and is greater than the clock pulse width at the 2R output, there was a problem that a digital error occurs. On the other hand, in a consecutive data transmission (transmission system for consecutively transmitting data without dividing data into packets or cells) the pulse width is compensated for.
FIG. 1 shows the pulse width compensation circuit for a consecutive data transmission.
In FIG. 1, the system is configured so that an amplitude regenerated digital output may be smoothed using a low-pass filter with a similar cut-off frequency to the transmission speeds and the degradation in the pulse width is compensated for by amplifying this output to a digital signal level again using a limiting amplifier having the mean value of the smoothed signal as a threshold.
That is, a signal with a reduction of amplitude, degradation of waveform, etc. which occur in a digital signal due to the losses, etc. during transmission, is inputted to a digital signal amplitude regenerator circuit 151. The digital signal amplitude regenerator circuit 151 amplifies the amplitude of the inputted digital signal, and compensates for the reduction of the amplitude so that the amplitude of the inputted digital signal may be amplified to the amplitude of the original digital signal. After being amplified up to the amplitude of the original digital signal, the digital signal is inputted to a low-pass filter 152, the high frequency component is removed, and it is smoothed. Then, the AC component only is extracted by a capacitor 155, and a DC component generated by a power supply 156 is added to the AC component. The digital signal processed in this manner is branched into two, with one signal inputted to a limiting amplifier 154, and the other signal to a mean value detector circuit 153. The mean value detector circuit 153 detects a mean value signal level of the digital signal processed as described above, and inputs the mean value to a limiting amplifier 154 as a threshold. If the digital signal directly inputted from the capacitor 155 is bigger than the threshold inputted from the mean value detector circuit 153, the limiting amplifier 154, for example, amplifies the digital signal to a signal level indicating a logical xe2x80x9c1xe2x80x9d. If the digital signal is smaller than the threshold, the limiting amplifier 154 reshapes the signal so that the digital signal becomes a signal level indicating a logical xe2x80x9c0xe2x80x9d.
If the configuration of the pulse width compensation circuit for the conventional consecutive data is applied to a burst data transmission, the above-mentioned mean value is biased to a aside in which no signal is present (the same code side: to the level of constant value) and proper pulse width compensation cannot be made for a time when data are not transmitted (or the same codes continue) for a long time. There is also a problem that a proper pulse width cannot be set, since the mean value also fluctuates according to the mark-to-space ratio.
It is an object of the present invention to provide a pulse width compensation circuit for compensating for the degradation of a pulse width, after a burst data signal is amplified to an original digital level.
The pulse width compensation circuit in the first aspect of the present invention is a pulse width compensation circuit for compensating for the degradation in pulse width of burst data, comprising a header area of a predetermined mark-to-space ratio, and a data area for storing data, and is characterized in comprising an initial potential generator circuit for providing an initial potential when detecting a threshold for compensating for the degradation in pulse width, a switch for switching the initial potential to a burst data signal when a burst data signal is inputted, a low-pass filter with a similar cut-off frequency to the signal transmission speed, for smoothing the output waveform from the switch, a limiting amplifier for amplifying the output of the above-mentioned low-pass filter and inputting it to a signal input terminal, and a threshold generator circuit for starting a threshold detection for compensating for the degradation in pulse width by switching the switch from the initial potential to the burst data signal when a burst data signal is inputted, terminating the threshold detection before the detection of the header area of the burst data signal is completed, and storing the threshold until the detection of the burst data signal is completed. The burst pulse width compensation circuit is also characterized in connecting the output of the threshold generator circuit to the reference potential terminal of the limiting amplifier, and compensating for the degradation in the pulse width of the burst data signal with the potential inputted to the reference potential terminal as the threshold.
The pulse width compensation circuit in the second aspect of the present invention is a pulse width compensation circuit for compensating for the degradation in pulse width of burst data, and is characterized in comprising a threshold generator circuit for detecting a threshold to compensate for the degradation in pulse width from the transmitted burst data signal, a pulse width compensation circuit for reshaping the pulse width based on a threshold generated by the threshold generator circuit, and an initial potential generator circuit for providing an initial potential when the threshold generator circuit detects a threshold.
The pulse compensation method of the present invention is a pulse width compensation method for compensating for the degradation in pulse width of burst data with a header area of a mark-to-space ratio of 1/2 and a data area for storing data, and is characterized in comprising a step of providing an initial potential when starting a threshold detection for compensating for the degradation in pulse width, a step of switching the initial potential to a burst data signal when a burst data signal is inputted, a step of removing high frequency components for smoothing the waveform of the burst data signal, a step of starting threshold detection for compensating for the degradation in pulse width when inputting a burst data signal, terminating the threshold detection before the detection of the header area of the burst data signal is completed, and storing the threshold until the detection of the burst data signal is completed, and a step of reshaping the pulse width of the smoothed burst data signal based on the threshold.
According to the present invention, since a circuit for compensating for the degradation in pulse width is supplied with an initial potential of a predetermined potential when there is no burst data signal, the setting detection of a threshold used to compensate for the degradation in pulse width can be started from an appropriate potential when a burst data signal is received. That is, if the setting detection of the threshold is performed to compensate for the degradation in pulse width even when there is no burst pulse signal, the threshold often becomes very low because there is noise when there is no burst pulse signal. Therefore, the threshold must be set high when a burst pulse signal is received in this state. Then, the detection goes beyond the header area of the burst data signal before an appropriate threshold is detected, if the detection speed of the threshold detection is slow, and thereby the degradation in pulse width of the data area cannot be appropriately compensated for. If the threshold detection speed is increased, the threshold violently fluctuates on the arrival of a burst data signal, and thereby an accurate threshold cannot be detected. On the other hand, according to the present invention, since an initial potential of a predetermined value is supplied when there is no burst data signal, a sufficiently accurate threshold can be detected within the header area of the burst data signal even if the detection speed is slowed down in order to improve the detection accuracy of the threshold. Accordingly, the degradation in pulse width of the succeeding data area can be appropriately compensated for.